Hello thouts
Are the signs you are trying to photograph internally lit, or externally? i.e. Neon signs would be internally lit, and billboard with a flood-light on it would be externally lit. Using a camera flash would be externally lit.
Do you need the background as well as the sign to be properly exposed? i.e. If you have a billboard on a building, are you photographing the building as well as the sign to show context, or are you just needing a photo of the sign itself?
The easy answer is: You are overexposing the signs. If you are getting white instead of colour, then you are putting way too much light onto the sensor.
Now you will understand why a professional photographer gets paid for what they do.
Your best bet is to get very close to the sign, and meter with both brightly lit and dark parts of the sign in the viewfinder. If you are going to handhold, keep the shutter speed number at a number that is at least as big as the focal length number. If you can get the shutter speed number bigger (its a fraction, so a bigger number is less time - so shorter shutter speed) you will find your photos are better. Using a tripod allows you use longer shutter speeds, but don't joggle the camera - use a remote release if possible.
Now, change the f/stop number (aperture) until the sign looks properly exposed. If you are getting white instead of colours, then close the aperture (i.e. bigger f/stop numbers) until the sign is properly exposed. Shoot in RAW if you can. RAW allows you to ignore the white balance/colour balance until you are back in the office. You should still be close to the sign. Now, record your shutter and aperture. Now you can go and stand anywhere you want, and that particular sign will still be properly exposed - as long as the light on it doesn't change. Exposure is entirely dependent on the distance between the light source and the subject. Camera distance does not matter - unless you are using an on-camera flash, where moving the camera is actually changing the light/subject distance. Any sign with similar lighting will have similar exposures. Once you have done a few, you may find you can just set up in the final location and skip the metering part.
Remember that bit about the pro photographer?
Now, if you need both the sign *and* the background properly exposed it gets complicated.
Traditionally, photographers would photograph these kinds of subjects at sunset and sunrise so that the light on the sign and the light on the background was more less the same. A bit of darker sky would really make the sign "pop". Now, you can use HDR techniques - but that is beyond the scope of this posting.
Remember, photographers aren't trained to make good signs - so sign makers may not be the best photographers. A good, old school, photographer with experience in shooting architecture could nail your sign photos in single sitting. If these are for advertizing purposes, a 4x5 photographer who knows what they are doing will give you a file that you could plaster over the side of a - well - bill board.
Good luck.
